This invention is concerned with a composite metal as a contact material for vacuum switches, which exhibits a heterogeneous microstructure and consists of at least two metal components.
In medium-voltage vacuum switches, pure alloys with a copper base or impregnated sintered materials are used as contact materials. These impregnated sintered materials consist of a porous, sintered matrix of a metal with a high melting point, which is impregnated with a metal or a metal alloy with a lower melting point and higher electric conductivity, so that a so-called composite penetration metal is produced. According to the accepted views (Electrical Times, 9, July 1970, "Vacuum Interrupters, Development and Applications", page 48), the contact materials used must have a low gas content and, in particular, an oxygen content of less than 1 ppm, so that upon melting or evaporation under the action of an arc no excessive pressure increase is produced in the switching tube. To meet these requirements, all heat treatment processes of the contact materials such as alloying, sintering or impregnating are performed in a high vacuum or in a reducing atmosphere with subsequent heat treatment in a high vacuum.
In spite of these precautionary measures, it is as a rule not possible to achieve an impregnation free of voids and pores with impregnated sintered material particularly with matrix metals having oxygen affinity such as silicon, mentioned in German Offenlegungsschrift No. 1,640,038 or chromium, mentioned in German Auslegeschrift No. 1,640,039. The reason for this is, that although with a highly porous matrix metal a decomposition of the oxide coating can be achieved without extreme requirements as to the purity of the protective gas, at high temperatures in a reducing atmosphere, these purity requirements are raised during the cooling phase so much that they cannot be met and unavoidable reoxidation occurs. In the instant impregnation process, the presence of oxide residues must therefore be expected. Because of the high impregnating temperatures, they are partially broken down as diffusion in the pores of the matrix metal filled with the impregnation metal is greatly inhibited. The oxide residue is partly separated off by the liquid impregnating metal and is taken along in the form of slag. With further penetration of the impregnating metal this leads ultimately to the formation of agglomerates in the matrix metal, so that entire pore areas of the matrix metal clog up due to oxide slag residue and are no longer accessible to impregnation. Composite penetration metals made in this manner therefore always contain, in addition to properly penetrated or impregnated areas, void and pore areas which contain oxide slag. The functional reliability of a vacuum switch, however, is greatly reduced by such accumulations of oxide residue, as fairly large amounts of gas, which can result in re-firing of the switch, are liberated through dissociation if the arc starts at such oxide residues.
Matrix metals such as tungsten, molybdenum, iron, cobalt and nickel, which are perfectly penetrated by impregnating metals such as, for instance, copper, are applicable, only to a limited extent for other reasons. Tungsten and molybdenum are not suited as matrix metals for interrupting currents above 10kA, which is caused essentially by the substantial electron emission that sets in. Iron, cobalt and nickel, on the other hand, exhibit considerable solubility for impregnating metals such as, for instance, copper, which results in a large drop of the conductivity of the contact material, so that the continuous current must be limited to undesirably low values in order to avoid excessive heating of the contacts.
With contact materials of copper alloys, which were previously mentioned, the difficulties with oxide residues remaining in the material do not exist, because the slag separates on the liquid melt and can readily be removed. Because of their large melting areas, such contact materials, however, tend to have an unfavorable burn-off behavior with interrupting power. Furthermore, due to the essentially homogeneous structure of these contact materials, a desired narrow statistical distribution of the breakoff current is not achieved, or is achieved only through such easily evaporating alloy additions which, because of their high vapor pressure, reduce the switching capacity in an unpermissible manner.
These statements explain why an optimal contact material for vacuum power switches, which meets the requirements of an oxygen content of less than 1 ppm, freedom from voids, low arc-breaking current with a narrow distribution curve, low weldability and a minimum conductivity of 5 m/ohm mm.sup.2 required for reasons of heating, has not yet been found to date.